1. Field of the Invention
This invention relates generally to waveguide used for transmission of broadband electromagnetic signals. More particularly, this invention relates to corrugated ridged waveguide of the flexible kind which can be processed in long lengths by a continuous process and has improved power-handling capability.
2. Description of the Prior Art
The use of smooth-walled waveguide is extremely common in microwave transmission systems. Waveguide of rectangular cross-section, in particular, is most often employed because it provides satisfactory electrical performance for a number of waveguide applications. Rigid and smooth waveguide, however, is subject to severe restraints, both economic and utility-based, because the non-flexible nature of such waveguide entails manufacturing in relatively short lengths and requires use of customized lengths, bends and twist sections to suit the equipment layout at each site. In many applications, therefore, waveguide which is rendered flexible by provision of corrugations is used. Such waveguide is commercially fabricated by first forming a smooth-walled tube from a tube of conductive metal and thereafter corrugating the tube.
In applications needing bandwidths greater than can be obtained from rectangular waveguide, some form of ridged waveguide, typically double-ridge waveguide, is used. In such ridged waveguide, ridges realize a perturbation of the cross-section which provides broader bandwidth between the cut-off frequency of the dominant-mode and the first higher-order mode. However, there are certain disadvantages inherent with the use of double-ridge waveguide. For instance, rectangular double-ridge waveguide, is problematic because the presence of a plurality of corners leads to substantial signal attenuation and the peak-power-handling capability of the waveguide is generally lowered. The sharp corners are also a source of problems in certain manufacturing processes such as electroplating.
Double-ridge waveguide of the rigid type is also disadvantageous in that it requires precise alignment with the system components in order to function effectively. The lack of flexibility of rigid waveguide also poses significant difficulties in handling, storage, and shipping. Rigid waveguide is particularly difficult to install and requires accessory coupling components even if the system sections to be linked by the waveguide are slightly displaced axially. More significantly, it is difficult to economically manufacture rigid double-ridge waveguide in long lengths through continuous processing techniques.
In applications where both flexibility and broadband operation are essential, such as in many defense-related applications like airborne cabling operations, radar jamming aboard military aircraft, etc., flexible double-ridge waveguide, typically of rectangular cross-section, is used. Flexibility is provided by means of successively formed corrugations of the desired double-ridge cross-sectional shape. The manufacturing process involved in fabricating such waveguide is expensive and time consuming because the corrugations are generally non-continuous and have to be formed individually. A major disadvantage is that continuous processing is not possible and, accordingly, flexible double-ridge waveguide is commonly available in short lengths only.
Although the presence of ridges yields increased bandwidth, the other electrical characteristics of ridged waveguide are degraded in comparison with rigid non-ridged waveguide of comparable length.
The attenuation factor is increased and voltage-standing-wave-ratios (VSWRs) are degraded to the point where satisfactory performance can be achieved only in very short lengths. Inherent with the use of short lengths are problems associated with the need for coupling flanges and the associated dry air/gas leakage, potential for intermodulation, resultant VSWR degradation, and need for providing mechanical access to the coupled lengths for alignment purposes.
Consequently, there exists a need for flexible waveguide having acceptable electrical characteristics, particularly high power-handling capability, suited for use in broadband dominant-mode microwave transmission applications and which can be economically manufactured in long lengths by a continuous process.